1. Field of the Invention
The present invention relates to a reflective projection screen for image, and more particularly, to a method of fabricating a reflective projection screen for a projector having a short focal length of a short projection distance and a shape thereof, which is applicable to 2D and 3D imaging. In particular, embodiments of the present invention relates to a reflective projection screen having multi-incident angles wherein every reflection surface is prepared per projection angle of projection image and incident angles are formed differently from each reflection surfaces and further the reflection surface has no reflection layer so that the projection images projected from different angles are reflected into one direction.
2. Description of the Related Art
Generally, a 2 dimensional (2D) image refers to a plain moving picture or a video image. Meanwhile, a 3 dimensional (3D) image refers to a cubic image, where there are typically two ways of showing the cubic image using a projector. In one way, polarized images are projected through left-right polarizing plates provided before projection lens of left and right projectors and the images are viewed through 3D glasses provided with left and right polarizing plates. The other way uses a glass-shutter where left image and right images are projected alternatively in sequence from a projector which is then received by 3D glasses. The alternating signals received by the 3D glasses controls the alternating opening and closing of the left and right glass lens based on the left and right images on a screen and synchronizing signal in the classes to view the cubic image.
However, referring to the polarizing manner as described above the image has to be sent to the polarizing glasses in a state of polarization to maintain a 3D image and further, referring to the glass shutter manner, the shutter is driven to view the 3D image such that the signal, such as infrared light from a projector, has to be projected to a screen and reflected from the screen to reach a infrared light sensor provided in the 3D glass. For this reason, referring to the polarizing manner, which is usually used in a theater, the screen for receiving the polarized image is extremely limited and thus 2D and 3D theater screens have to be separately prepared due to the light reflection of the screen.
When the images are polarized, only 25% of image light is transmitted through a polarizing plate of a projector and 25% of the image light is transmitted through 3D glass and thus only 12.5% of the image light is viewed on the screen. Accordingly, the polarization degree decreases to 12.5%.
Meanwhile, when the 3D image is viewed using shutter plates, since left and right images are projected in sequence alternatively from a projector, 50% of the image light is transmitted, having a transmission rate of 50%, and 30% of the image light is transmitted through the shutter glasses and thus 15% of the image light is viewed on a screen. Accordingly, the infrared light signal for synchronizing the shutter decreases dramatically to 15%. For this reason, the brightness of the 3D image decreases which dramatically lowers the definition thereof and thus the 3 dimensional feeling. Furthermore, when a user views the dark image for a long time, the user's eyes would become tired and the user develops a headache. Thus there is need to develop brighter screen.
U.S. Pat. No. 7,227,683 and U.S. Pat. No. 7,362,502, which were filed by the present applicant, disclose the use of the reflective screen and spherical screen which are considered to be 20 times brighter than conventional screens wherein as shown in FIG. 4 of the present application, when a projector is placed on a focal point (f) of a spherical screen 1a, the image is reflected straightly from the spherical screen 1a and thus a reverse angle range (e) is enlarged to fit an entire screen size, thereby diminishing a hot spot appearance and increasing brightness uniformity.
The spherical screen 1a is applied efficiently to a 3D cubic image as well as a 2D image, however, when a projector of a short focal distance is used, a radius of curvature formed by a short focal point limits the screen size. That is, as shown in FIG. 5, a focal distance (projection distance) f is r/2, and thus r=2f. Accordingly, when f becomes shorter, r becomes smaller; therefore, the screen size g also becomes smaller. The screen size is determined within the range g formed by 2 times curvature having the projection distance of the projector having a short focal distance point, which projects in a short distance, and thus the picture size decreases as much as the projection distance is shortened, and makes it impossible to have a larger size screen configuration.
Additionally, referring to the spherical screen a thin film screen which is rolled up, i.e., rollable, such as a roll screen, cannot be used due to a curvature depth of a spherical surface. For example, a screen configuration using Fresnel reflection lenses for refracting and reflecting images using a prism configuration, the image has to be transmitted through the prism medium for both refraction and reflection even when a reflection surface is provided on a rear surface thereof. Here, it has been also disclosed that when the image is transmitted through the prism medium, a refraction rate of polarization degree varies and thus it cannot be applied to 3D image. In particular, in the Fresnel configuration, as shown in FIG. 14, it has been disclosed in “Optical” in page 374 (translation by Kwon Yong Dae) that the polarization degree decreases by a half-wave from point p and thus a picture size cannot be enlarged in contrast to the focal point distance. Accordingly, the screen thickness may be formed as a thin film in the Fresnel configuration, however, the larger screen than the focal point distance cannot be fabricated.
Therefore, the screen using Fresnel reflection, as shown in FIG. 6, cannot be applied with a projector having a short focal distance under a large screen.
That is, since the projector having a short focal distance has to be placed on a middle axis of a Fresnel reflection screen for normal operation, i.e., it has to be applied to a transmissive screen. However, when the projector having a short focal distance is placed on a middle part of a screen, view of the image is blocked and thus is not practical. In particular, when the image is transmitted through the transmissive medium of the Fresnel screen, the polarization degree of the cubic 3D image collapses and thus it prevents the use of the Fresnel screen as a 3D screen.
Additionally, Korean Patent Application No. 10-2003-0051853, which was filed by the present applicant, discloses a high refraction reflective screen, as shown in FIG. 7 wherein the images incident from several angles are not reflected in a straight manner at the same angle but reflected in a diffuse manner to the incident direction. Accordingly, reverse angle e of a reflection angle to a screen height is small and thus a hot spot phenomenon in which a brightening of a part of the screen prominently occurs. Additionally, reflection angles are diffused depending on incident angles and it acts as a mirror which changes reflection direction and thus brightness uniformity is poor and picture brightness is different depending on the reflection angle, thereby making it impossible to be used as a screen. In particular, a polarization degree of polarized 3D image is diffused and thus it can be used only finitely as a 3D cubic screen.
Meanwhile, referring to a projection distance of a general projector, the projection distance is 4-5 m based on a lateral length 2 m of the screen and thus a ratio of the projection distance to the lateral length of the screen is 2-2.5: 1.
However, when the projection distance becomes longer as described above, there arises a problem in that the projector and the screen are arranged separately. Further, the prior projector having a long projection distance occupies a large installation area, thereby making it difficult to install at home.